WASHINGTON (CNN) -- The Pentagon may send 3,000 Marines to Afghanistan in the coming weeks to reinforce the country's British-led sector ahead of an expected spring offensive by the Taliban and al Qaeda.

French soldiers with the NATO-led International Security Assistance Force patrol the streets of Kabul on Tuesday.

U.S. Army Gen. Dan McNeill -- the top NATO commander in Afghanistan -- made the request.

As of Wednesday, Defense Secretary Robert Gates was "giving it a hard look," according to Pentagon spokesman Geoff Morrell.

Central Command and the Joint Chiefs of Staff have already approved the request.

Gates could follow suit very quickly after finishing discussions with his commanders, Pentagon officials said.

"The request is based upon an anticipated spring offensive by the Taliban. They failed last time, and they will fail again this time, but commanders are seeking additional forces to ensure that," Morrell told CNN, signaling that the request is expected to be approved.

The Marines would be sent on a seven-month tour. It's considered a "major commitment" of forces, according to a Pentagon official. There are about 26,000 U.S. troops in Afghanistan, and the 3,000 additional forces would boost U.S. troop levels there to their highest point since the 2001 invasion.

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Under the proposal, the bulk of the Marines would go to southern Afghanistan, where British and Canadian forces have been in heavy combat for months. Others may be assigned to training Afghan forces.

NATO has been looking for a commitment of 7,000 additional forces for both combat and training missions.

But member countries of the alliance have failed to meet that goal, and it had been expected for the last several weeks that McNeill would be forced to turn to the Pentagon for more forces.E-mail to a friend